The great re-freezing

The ice on Birch Lake went out on March 14. Eight days later we woke to a new skin of ice over nearly the entire surface. As I write on March 28, the ice remains and is covered with almost a foot of snow. This is the first year in which we will have to note two ice-out dates on the board beside the refrigerator where we keep such records. Most likely the experience on your lake has been similar.  It is unusual for lakes to re-freeze, but a cold spell of several days, with a couple of nights of no wind and single-digit temperatures, did the trick. When I went down to the lake to investigate, I observed a fragile skin out toward the middle, but thicker ice near shore.

To test it I hurled a stone half the size of my fist as high as I could, thinking it would punch right through. Instead it hit with a thock!, punching a hole and rolling off to the side. On that basis I guessed the thickness at three-quarter of an inch. I wasn’t inclined to verify by walking on out. A week later, since the cold has persisted, we likely have a more robust ice cover extending most of the way across, although I won’t be pulling the ice fishing gear back out of storage.

Lakes usually don’t re-freeze because the ice goes out, on average, in mid-April, when seasonal temperatures are fairly mild. As the ice gradually melts we’ll have high 40s or low 50s during the day, and nights still above freezing. Once the ice cover is gone the, water warms rapidly, and there’s no turning back, even if a deep and fairly persistent cold front moves through. For re-icing to occur, the water temperature would have to drop to the freezing point of 32 degrees and then, much more significant, give up the heat of fusion. It takes 80 times as much cooling to make ice from water that has reached the freezing point as it would to lower the temperature of that water by one degree Celsius.

This time, because the ice went out so early, the days and especially nights were still pretty cold. So the water was hanging out just slightly the above the freezing point – and then came the cold snap. It likely won’t take much warm weather to open the lakes again. Normally at this time of year we still have a foot or more of ice; now we have at most a couple of inches, and some open areas near shore where springs bubble in. Once the sun goes to work on that and any other near-shore openings, it will heat up the bottom sediment, in turn warming the water, and the process of melting the ice from below will have begun. I’d like to see open water again by mid-April and look forward to installing the pier and getting the pontoon boat in the water.